Bachy Soletanche is in the middle of one of the biggest foundation contracting jobs in Europe
The Tyne Tunnel crossing in Newcastle is worth £37 million for French-based firm Bachy Soletanche and the huge scheme includes piling and diaphragm wall construction for the tunnel portals. This second tunnel being built by the Tyne & Wear Passenger Transport Authority. The new crossing will run to the east of the existing ones and includes a road tunnel and a pedestrian and cycle tunnel.
Work started on the north side of the river in August 2008, while work on the smaller site south side of the river began in September 2008. On the north side work could begin immediately on the diaphragm wall, but on the south side the team installed 320 CFA piles with 300 mm to 1 m diameter.
All these works largely cover the construction of retaining walls for about 1.1 km of cut and cover tunnel to build the portal sections on the approaches to the submerged tunnels which are being placed by Volker Stevin.
Varying ground conditions
The work at the new Tyne tunnel is not only demanding because of the ground conditions so near to the river bank, but the team have also had to bring a large number of rigs to the site to allow for the variations in soil. Bachy Soletanche project manager Claire Doby explains: “One of the biggest challenges we’ve had is bringing all the different equipment here. This is a big project but the time we have to complete the job is not very long, we have until the end of July. So that means we have lots of contractors working on site, creating a large facility in a restricted area.”
On the south side, where the piling took place, the ground shifted from made ground to clay, mudstone and sandstone.
The diaphragm wall techniques also varied with ground conditions, including rope grab, hydraulic grabs, together with a hydrofraise latine mill. The diaphragm wall rigs are supported with CFA and rotary piling rigs to construct the secant and contiguous bored pile rigs. On site there are three robe grab rigs, four hydraulic grabs, one hydrofraise latine mill and one drill.
“In addition to this equipment we have three de-sanding plants, which are being used by our concrete suppliers. On the south site we have Cemex as our partner and on the north Tarmac,” adds Ms Doby.
The concrete suppliers are bringing a high slump mixture to the project – A4S4 type concrete.
Going on to explain the diaphragm wall construction Ms Doby says: “On the north side we have a diaphragm wall of more than 24,000 sq m and on the south side it’s more than 27,000 sq m.
“In total that’s around 51,000 sq m of diaphragm wall, all 1.2 metres thick.”
The project also requires the use of three yards to fix reinforcements – one each on the north and south banks of the river, with a third smaller yard near Edinburgh. Ms Doby says that this added to the logistics challenges of the job: “We have 100 people on site, including our own management and the sub-contractors. Adding to that is the plant, where we require a wagon for each grab to remove the spoil from the site.”
Most of the staff are from the UK, but additional rigs from Bachy Soletanche’s fleet in France and Spain have been sent across to assist the UK fleet and ensure it’s afull strength and is up and running at all times.
When the new tunnel is completed, the existing 40 year-old tunnel will be refurbished and a new interchange will replace the existing junction at the southern end of the Tyne Tunnel, allowing a new toll plaza to be installed on the north side of the crossing.
It is hoped that both vehicle tunnels will be fully operational by December 2011.